1. Technical Field
This invention relates to a power circuit, a liquid crystal display device incorporating this power circuit, and electronic equipment incorporating either the power circuit or the liquid crystal display device.
2. Related Art
FIG. 18 is a schematic diagram of electrodes of a liquid crystal panel that is multiplex driven. To simplify the description, this liquid crystal panel includes nine segment electrodes and six common electrodes. Each rectangle in this figure represents an electrode formed in the liquid crystal panel, and these electrodes are divided into segment electrodes (SEG1 to SEG9) and common electrodes (COM1 to COM6), depending on the signals that are applied thereto. The square shaded portion at each intersection of electrodes represents a display dot. To ensure that each display dot operates as a capacitor, a capacitor links the segment electrode and common electrode at a corresponding display dot.
Voltage levels V0 to V5 that are necessary for driving liquid crystal elements by a multiplex-driven method with a high duty ratio (six-level drive method) generally satisfy the relationships V0-V1=V1-V2 =V3-V4=V4-V5 and V0&gt;V1&gt;V2&gt;V3&gt;V4&gt;V5, as shown in FIG. 19. In this case, it is assumed that V0 to V2 are a first group of drive voltages and V3 to V5 are a second group of drive voltages. The voltage range of the first group of drive voltages V0 to V2 does not overlap the voltage range of the second group of drive voltages V3 to V5, and is separate therefrom.
FIG. 19 shows examples of signal waveforms applied to the segment and common electrodes. The signal applied to a segment electrode generally switches between voltage levels V3 and V5 during the period of a frame 0 (hereinafter called FR0). Similarly, the signal switches between voltage levels V0 and V2 during the period of a frame 1 (hereinafter called FR1). This switching of the voltage level at the segment electrode depends on the pattern to be displayed.
At the same time, the signal applied to a common electrode is at voltage level V4 during a non-selected state in the FR0 period, or at voltage level V0 during a selected state. Similarly, the signal is at voltage level V1 during the non-selected state in the FR1 period, or at voltage level V5 during the selected state. The period during which a common electrode is in the selected state differs for all of the common electrodes, and it is not possible for a plurality of common electrodes to be simultaneously selected. Voltage levels are inverted between the FR0 and the FR1 periods, to drive the liquid crystal in an alternating fashion.
The switching of the voltage levels at the segment and common electrodes is in accordance with the charging and discharging of capacitances existing within the liquid crystal panel. This means that currents flow through the liquid crystal panel between each of the voltage levels V0 to V5. These currents are hereinafter called panel currents.
The voltage level of each segment electrode switches between V0 and V2 (in the FR1 period) or V3 and V5 (in the FR0 period). For a large proportion of the time, each common electrode is in a non-selected state, so the voltage level thereof is mostly at V1 (in the FR1 period) and at V4 (in the FR0 period). This means that a panel current caused by a switch in voltage level at a segment electrode (hereinafter called a segment panel current) flows mainly between the first group of drive voltages V0, V1, and V2 or the second group of drive voltages V3, V4 and V5. Similarly, a panel current caused by a switch in voltage level at a common electrode (hereinafter called a common panel current) flows mainly between V5 and a voltage within the first group of drive voltages V0, V1, and V2, or between V0 and a voltage within the second group of drive voltages V3, V4, and V5.
A power circuit that supplies the above panel currents is disclosed as a prior-art method in Japanese Patent Application Laid-Open No. 2-150819. The configuration thereof is shown in FIG. 20. In this figure, external power sources VDD and VEE are provided to drive the power circuit, where VDD is greater than VEE. Divided voltage levels V1' to V4' are generated between VDD and VEE by series-connected resistors R1 to R5. These divided voltage levels are input to operational amplifiers OP1 to OP4 that are each connected in a voltage-follower configuration. The operational amplifiers OP1 to OP4 use VDD and VEE as power sources to convert V1' to V4' by lowering the high impedances thereof. C1 to C4 denote smoothing capacitors that control variations in the output voltage levels V1 to V4 of the operational amplifiers OP1 to OP4. Resistors R8 to R11 limit the output currents of the operational amplifiers OP1 to OP4, and thus reduce the power consumption thereof.
However, this prior-art method has problems, as described below.
The first problem with this prior-art method concerns the way in which a large amount of power is wasted when panel currents are supplied. The reason for this is as follows. When a liquid crystal panel is driven using this power circuit, panel currents are supplied as part of the current flowing from VDD to VEE. Consider, for example, a segment panel current flowing from voltage level V3 to V4. This segment panel current flows out initially from the power source VDD and into the liquid crystal panel through the operational amplifier OP3, as shown in FIG. 21. This current from the liquid crystal panel flows into the power source VEE through the operational amplifier OP4. When a segment panel current is supplied in this fashion from voltage level V3 to V4, the current flowing from VDD to V3 causes the operational amplifier OP3 to generate heat. Similarly, the current flowing from V4 to VEE causes the operational amplifier OP4 to generate heat. In other words, these currents do not operate efficiently in the driving of the liquid crystal panel. All the other panel currents are equally inefficient. This means that the use of the power circuit of FIG. 20 for supplying the common panel currents and segment panel currents leads to wasteful consumption of power as heat generation of the operational amplifiers. In this particular example, the common panel currents mainly flow between V5 and a voltage within the first group of drive voltages V0, V1 and V2, or between V0 and a voltage within the second group of drive voltages V3, V4 and V5. Therefore, there is a large voltage difference between the voltage levels that these currents flow between, and there is only a small difference between these voltages and the voltage between the power sources VDD and VEE. In contrast, the segment panel currents mainly flow between voltages within the first group of drive voltages V0, V1 and V2, or between voltages within the second group of drive voltages V3, V4 and V5. Therefore, there is a small voltage difference between the voltage levels that these currents flow between, and there is a large difference between these voltages and the voltage between the power sources VDD and VEE. Thus it is clear that the proportion of power that is wasted by such components as the operational amplifiers is greater, when supplying a segment panel current than supplying a common panel current. In other words, in order to prevent such waste of power, it is necessary to improve the method used to supply segment panel currents.
The second problem with this prior-art method concerns the large power consumption caused by idling currents in the operational amplifiers. In other words, the operational amplifiers OP1 to OP4 of the prior art are driven by the power sources VDD and VEE, as shown in FIG. 20. However, since these power sources VDD and VEE are also used as power sources for generating the drive voltages for the liquid crystal panel, the voltage between VDD and VEE is extremely large. For a constant current, power consumption is proportional to voltage. Therefore, if the voltage between VDD and VEE increases, the power wasted by the idling currents in the operational amplifiers OP1 to OP4 also increases.
The third problem with this prior-art method concerns the way in which the above described use of power sources VDD and VEE with a large voltage difference therebetween requires expensive operational amplifiers that can withstand such high voltages.
To design a liquid crystal display device with a higher duty ratio, it is generally necessary to increase the voltage between the power sources VDD and VEE even further. This would further aggravate the above described three problems.
Note that another prior-art method for a power circuit that supplies power to a liquid crystal panel is disclosed in Japanese Patent Application Laid-Open No. 3-200214, whereby a switched capacitor circuit is used to generate voltage level V5 from V1 and voltage level V4 from V2. However, this prior-art method has the objectives of reducing the number of operational amplifiers or the amount of wiring between the power circuit and the liquid crystal panel for implementing a smaller device. Nevertheless, the prior art does not have the objective of reducing the power consumption, unlike the present invention.
In addition, this prior-art method has a five-level output wherein the voltage of a common electrode when it is not selected is the same (V3=GND) before and after each polarity inversion, in other words, during the FR0 and FR1 periods of FIG. 22(A). In order to increase duty ratio for driving the liquid crystal with this power source configuration, it is necessary to increase the voltages between V1 and V3 and between V3 and V5, as shown in FIG. 22(B). This causes a problem in that it requires provision of a high-voltage-withstanding driver chip for driving the liquid crystal. Therefore, the power source configuration of FIG. 22(B) is generally used, when a liquid crystal device is to be driven at a low duty ratio. The power source configuration shown in FIG. 22(C) has been proposed for driving a liquid crystal device at a high duty ratio, to solve the problem generated by the power source configuration of FIG. 22(B). This method is generally used to drive a liquid crystal device at a high duty ratio. The present invention is aimed at the power source configuration shown in FIG. 22(C), but its objectives and configuration are different from those of the prior-art method that is designed for the power source configuration shown in FIG. 22(B).
The present invention is devised to resolve the above described technical concerns, and has as its objective the provision of an inexpensive, less power-consumption power circuit, liquid crystal display device and electronic equipment.